1. Field of the Invention
The present invention relates generally to beam index color television receivers, and, more particularly, is directed to a circuit for controlling the electron beam in a beam index color television receiver.
2. Description of the Prior Art
Beam index color television receivers are known which include a color cathode ray tube having a phosphor screen with red, green and blue color phosphor stripes arranged thereon in the horizontal line scanning direction and with phosphor index stripes arranged on the inner surface thereof also in the horizontal line scanning direction. As is well known, the color the phosphor stipes are arranged in RGB triads repetitively across the screen so as to be scanned by the single electron beam as the latter effects a horizontal line scan in, for example, left-to-right traverse. As the electron beam scans the color phosphor stripes, it also scans the index strips which emit light when excited by the scanning electron beam. A photo-detector responds to each excited phosphor index stripe to produce a signal whose frequency is equal to the frequency at which the phosphor index stripes are excited. Thus, as the electron beam scans a horizontal line across the display screen, the photo-detector generates a periodic index signal. The index signal which is derived from the scanning of the aforementioned phosphor index stripes is used to gate red, green and blue primary color signals onto, for example, the first grid of the cathode ray tube in successive time sequence. In particular, the electron beam is density-modulated with the red primary color signal when the electron beam scans each red phosphor stripe, with the green primary color signal when the beam scans each green phosphor stripe, and with the blue primary color signal when the beam scans each blue phosphor stripe, respectively. In such beam index color television receivers, it is known to arrange the index stripes so that three index stripes are provided for every two sets or triads of red, green and blue color phosphor stripes so that the pitch of the index phosphor stripes is not equal to an integer multiple of the pitch of one set or triad of color phosphor stripes. With such an arrangement, the phase of the index signal is not shifted with the varying excitation of the color phosphor stripes.
However, when the index stripes are so arranged, the gating of the primary color signals to modulate the scanning electron beam in respect to the respective color phosphor stripes becomes difficult. In such a case, the index signal is supplied to, for example, a PLL (phase-locked loop) circuit where it is frequency-multiplied to produce a signal having a frequency three times that of the so-called triplet frequency, the latter being determined by the pitch of one set or triad of color phosphor stripes and the scanning speed of the electron beam. However, when the signal having a frequency three times that of the triplex frequency is supplied to a one-third frequency divider to provide, for example, 3-phase gate signals for gating the respective primary color signals, to the control grid of the cathode ray tube it is necessary to provide a separate control means so that, upon initiation of the horizontal scanning operation, the respective gate signals have their phases aligned or matched to gate the primary color signals when the electron beam is positioned in respect to the corresponding color phosphor stripes.